1. Field of the Invention
The present invention relates an image processing method for processing an electronic image that is picked up from an original, so as to reduce moiré in the image.
2. Background Arts
An imaging system has been known in the art, that consists of image input devices, including a scanner, an electronic still camera or so-called digital camera and a memory reader, a personal computer and a digital printer. An electronic image entered through the image input device into the personal computer is processed synthesizing the image by use of a template or changing the magnification of the image. Based on the processed image data, the digital printer prints a hard copy of the image on a recording paper. As the digital printer, there are a silver halide photography type, a heat developing photosensitive material type, a thermal recording type, an ink jet type and so forth.
The reflective scanner projects light onto an original, such as a printed matter and a photo-print, and picks up reflected light from the original through a line sensor, to output an image signal. The line sensor consists of a large number of photoelectric sensor cells arranged at a regular spacing. It is known in the art that an image printed based on the image data from the reflective scanner sometimes contains moiré, and that the moiré is caused by a ratio of a spatial frequency of the image and a sampling frequency that is determined by the spacing between the photoelectric sensor cells.
The original may have a grayscale image and/or a text printed thereon, and the moiré will be more conspicuous in those portions of the image where there are fine grayscale patterns, and thus the spatial frequency is high. For this reason, it is conventional to use a low pass filter (LPF) through which gains of the picked up image signals are reduced in a higher spatial frequency band, for reducing the moiré in the reproduced image.
However, the conventional low pass filter reduces not only the high spatial frequency band but also middle and low spatial frequency bands, so that the sharpness is lowered in the portions of middle and low spatial frequencies, that is, the portions containing binary images, such as characters, letters, marks or lines. Hereinafter, these portions of middle and low spatial frequencies will be referred to as “characters” for the sake of avoiding redundancy. In order to avoid lowering the sharpness of the characters, it has been necessary to adjust characteristic curves of the low pass filter finely according to the content of the original. Such fine-adjustment of the low pass filter characteristic curves is so difficult that it requires a skill.